Add some simple write buffering to LibC's stdio.

Plumb it all the way to the VirtualConsole. Also fix /bin/cat to write()
the whole chunks we get from read() directly to stdout.

LibC/entry.cpp

@@ -1,4 +1,5 @@
 #include <stdio.h>
+#include <string.h>
 #include <Kernel/Syscall.h>
 #include <AK/StringImpl.h>
 
@@ -20,6 +21,8 @@ extern "C" int _start()
 
     errno = 0;
 
+    memset(__default_streams, 0, sizeof(__default_streams));
+
     __default_streams[0].fd = 0;
     stdin = &__default_streams[0];